The objective of this research is to study the viral and the cellular control of the transformed phenotype in cells transformed by the oncogenic viruses polyoma and SV40. We also want to study the control of viral DNA integration and of the expression of viral functions in the transformed cells. Our work is planned along three main lines of research: 1) The study of viral and cellular factors controlling the association between the viral and the cellular genome in rat cells transformed by polyoma virus. In this system, "free" and integrated viral DNA molecules coexist, and the free molecules result from excision and limited replication of the integrated viral DNA. Excision requires an active viral A-gene product, and generates "cured" cells, which have lost an integral number of viral DNA molecules. Amplification of integrated viral DNA can also occur in the presence of a functional large T antigen. 2) The study of the regulation of viral transcription and T antigen expression in SV40 transformed cells. Expression of viral functions appears in fact to be controlled by the position of the transformed cell in the mitotic cycle. 3) The study of the minimal amount of viral functions required for cell transformation by polyoma virus.